EES-3022 improve tabs accessibility

[amyb-hiveit]

Accessibility improvements for tabs:

• when there's only one tab, for instance when datablocks with just a table are embedded in release content, the tab list is aria-hidden to prevent screen readers from seeing it.
• added screen reader only text to the tabs so they make sense when read out of context. This is via a new tabLabel field as the title is used in GA events so had to remain just a string
• changed it so the up and down keys move between tabs and prevented being able to loop over the tabs with the keyboard, this makes it more consistent with expected tabs behaviour and the GDS design system

Related changes

• changed user clicks link to use xpath so that data block tab links can be clicked now that there is adjacent visually hidden text
• I noticed that the Headlines Key Stats block on releases has the title 'Headlines' in the admin and 'Summary' in the frontend, so have changed the admin one to be consistent with the frontend

[ntsim]

I'm surprised that the xpath selector is working compared to the link selector used by user clicks link. Would have thought these are basically identical in what they're doing.

If there is actually a difference here, would suggest scrapping this new keyword and change user clicks link to use the xpath selector instead. It's simpler and if it also covers cases that link can't, then it'd be more beneficial to be there instead.